In the manufacturing of a semiconductor device, a various kinds of films are formed on a wafer. After a film forming process, the wafer is polished in order to remove an unnecessary portion of a film or surface irregularities. Chemical mechanical polishing (CMP) is a typical wafer polishing technique. A CMP process is performed by rubbing a wafer against a polishing surface while supplying slurry onto the polishing surface. A film, which has been formed on the wafer, is polished by the combined effect of the mechanical action of abrasive grains contained in the slurry and the chemical action of a chemical component of the slurry.